We have demonstrated that certain murine tumors, e.g. BL6 melanoma, contain cell surface receptors for the basement membrane glycoprotein laminin and that metastases can be increased by preincubation of BL6 cells with laminin followed by intravenous injection in C57 mice. Conversely, preincubation of these same cells with a liminin fragment (C1) containing the receptor binding site, but lacking the globular end regions necessary for binding to vascular basement membrane, inhibits metastases. Human breast epithelium rests on a laminin containing basement membrane, whereas invasive breast carcinoma loses its extracellular basement membrane. We have demonstrated an exposed laminin receptor in human breast carcinoma and localized it to the cell surface of the invasive tumor cells.